1. Field of the Invention
The present invention relates to a mixture of ethylenically unsaturated polyurethanes, to one-component coating compositions containing these polyurethanes as binders and to the use of these compositions for preparing coated substrates.
2. Description of the Prior Art
Low viscosity, aliphatic polyurethanes that contain acryloyl groups are known and described in U.S. Pat. Nos. 5,739,251 and 5,767,220. These polyurethanes can be formulated without the use of significant amounts of organic solvents or reactive diluents, which are environmentally objectionable and regulated by the government.
Coatings prepared by curing these polyurethanes, e.g., in the presence of UV light, possess many valuable properties, including hardness, solvent resistance and surface appearance. However, additional properties are necessary in order for these coating compositions to be suitable for multi-coat applications, such as automotive or wood coatings. For example, it must be possible to sand the surface before the application of the next coating in order to improve intercoat adhesion. Sanding abrades the surface, which provides additional bonding sites for the next coating.
One of the deficiencies of coatings prepared from ethylenically unsaturated polyurethanes is that they are very difficult to overcoat, which is primarily due to the fact that these coatings are not sandable. Either too much energy is necessary to abrade the surface or a phenomenon known as sandpaper loading takes place.
Accordingly, it is an object of the present invention to provide ethylenically unsaturated polyurethanes that possess improved sandability such that they are suitable for multi-coat applications. It is an additional object of the present invention to improve the sandability of these coatings, while also retaining the other valuable properties of the prior art coatings.
This object can be achieved by using the mixtures of ethylenically unsaturated polyurethanes according to the invention which are described in detail hereinafter. The fact that coatings prepared from the inventive mixtures possess improved sandability is surprising since individually neither resin possess this property.